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The package xxl.core.math.statistics.parametric.aggregates provides implementations of the techniques proposed in [HHW97]: J. Hellerstein, P. Haas, and
H.&nbsp;Wang, 1997. There, parametric estimators are computed online over a data stream and in some cases they are even equipped with the according
confidence intervals. The online computation of the aggregation functions relies on the {@link xxl.core.cursors.mappers.Aggregator aggregator cursor}.
An aggregate is computed by an aggregation function inductively with agg_n = f (agg_n-1, next).
This means an aggregation function must be a two-figured function expecting as first argument the previous computed aggregation value and as second argument the next object to use for 
computation. 
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